Gearwheel and method for manufacturing a gearwheel

ABSTRACT

The invention concerns a gearwheel ( 1 ) with a gear rim ( 2 ) and an insert ( 3 ) whose external diameter (d 3 ) is greater than half of the external diameter (d 1 ) of the gearwheel ( 1 ), wherein the insert ( 3 ) is surrounded by injection molding by means of a cone mold during the manufacture of the gearwheel. In particular, the gear rim ( 2 ), including gear teeth on the external circumference, is formed at the same time during the injection molding process.

The invention concerns a gearwheel with the generic characteristics ofclaim 1 and a method for manufacturing a gearwheel with the genericcharacteristics of claim 11.

Gearwheels of the prior art include, for example from the field of powersteering, so-called EPAS (EPAS: Electric Power Assisted Steering)gearwheels that have as a central element a large metal hub that isbonded with an external molded polyamide ring. A large metal hub in thisinstance is understood to refer to a metal hub whose external diameteris greater than half of the exterior diameter of the gearwheel. Gearteeth are then formed on the gear rim through machining of the moldedpolyamide ring material. This type of manufacturing process is expensivebecause a large number of expensive work steps are required. Anotherdisadvantage is the fact that a bonded connection between the externalcircumference of the metal hub forming the insert of such a gearwheeland the gear rim surrounding the insert provides only a conditionallysolid fit in the event of impacting forces in the circumferential orrotational direction and in directions parallel to the axis.

Surrounding an insert by injection molding with plastic whenmanufacturing a gearwheel in order to form the gear rim is also of theprior art. In particular, if the gear teeth are formed at the same timeduring the process, then the gear teeth are prevented from freelyshrinking when the molded plastic is cooled, which places the plasticring under permanent tensile stress. A large insert also largely reducesshrinkage tension if it is surrounded with plastic through injectionmolding. Such shrinkage tensions have an effect in the radial and axialdirection, and in particular generate the risk of gear breakage.

FIG. 9 shows a perspective view and a sectional view of an insert basedon the state of the art, in which protrusions form in thecircumferential area in the radial direction of the insert, which in themanner of a gear tooth are intended to positively engage with asurrounding material of a gear ring. FIG. 10 shows two other relatedsectional views that also show a gear ring. As is evident from FIG. 11,if material is injected from a gate located in an axial direction of thegearwheel being formed, material would flow out around such 34°protrusions. However, when the casting material is cooled or hardened, afissure S between the gear rim 2 formed in this manner and the insert 3*is created on the side of the 34° protrusions facing on the sideopposite the molding injection point.

The objective of the invention is to provide a gearwheel with a simplestructure whose individual components are also well attached.Accordingly, a method for manufacturing a gearwheel shall also beproposed that permits the production of such a gearwheel.

This objective is achieved by a gearwheel with the characteristics ofclaim 1 and by a method to manufacture a gearwheel with thecharacteristics of claim 11. The dependent claims involve advantageousembodiments.

In particular, therefore, a gearwheel is preferred with a gear rim andwith an insert whose exterior diameter is greater than half of theexternal diameter of the gearwheel, with the insert being surrounded byinjection molding using a cone mold or surrounded by injection moldingduring manufacture in accordance with the method.

It is clear that such a connecting part permits counter-grippingstructures that provide especially strong engagement to opposerotational forces and forces parallel to the axis due to a deeppenetration into the external part and the insert. This allowscounter-gripping structures that penetrate deeper into the external partand into the insert than is possible with conventional individualcomponents, which are inserted into one another based upon theirspecific manufacture.

The gear rim is (preferably) formed by means of the cone mold as acomponent surrounding the insert. Compared to the injection molding ofthe prior art, this avoids seams that would be created if plastic wereinjected from several injection points located in a radial directionaround the circumference. The lack of such seams results in increasedstability in the rotational direction of such a preferred gearwheel.

The gear rim (including an external gear structure) is preferably formedby means of the cone mold. Forming the external gear structure togetherwith the elements of the gear rim obviates the need for a subsequentmilling process to create gear teeth from a solid piece, and thusobviates the need for a completely independent manufacturing step.

The insert can have indentations facing outwards toward the gear riminto which the gear rim material penetrates. Such indentations are thusexternal; i.e., they are formed on a radial exterior side of the insertfacing the gear rim. At least one part of the indentations is preferablyformed as a globoid-like indentation. Such globoid-like indentations inparticular permit especially effective inflows of gear rim material,which also prevents formation of fissures between gear rim and insert.

The insert preferably has off-center protrusions on the exterior of thegear rim, which penetrate into the material of the gear rim. Suchprotrusions are external; i.e., they are formed on a radial exteriorside of the insert facing the gear rim. Preferably at least one part ofthe off-center protrusions extends with an oblong extension transverseto a rotational direction. This permits a large positive contact surfacefor good force transfer between the gear rim and the insert. At leastone part of such off-center protrusions is preferably displacedoff-center to the gate side of the cone mold.

The insert is preferably made from metal and the external part fromplastic. Alternatively, the insert can also be surrounded by injectionmolding with a metallic substance.

It is preferable to create a lateral molded part by means of the conemold on the side of an insert wall in the axial direction of the insert.A lateral molded part created in the axial direction by means of thecone mold is preferably then removed. The gear rim (including anexternal gear structure) is preferably already formed by means of a conemold.

However, a gearwheel can also be formed with an external part formed bythe gear rim, with an insert, and with one or more connecting elementsto create a form-fitting connection between the external part and theinsert, whereby the (at minimum one) connecting element is formed from amaterial that is injected between the external part and the insert.

The components that are injection molded by means of a cone mold,preferably with gear tooth structures that prevent pulling in therotational direction and/or in a direction parallel to the axis,preferably engage with the insert and/or the external part, whichcreates good rotational stability and/or stability against shiftingparallel to the axis for the insert and the external part relative toone another. Such gear tooth structures can be formed especiallyadvantageously by indentations and by protrusions placed off-center onthe external circumference of the insert, which permits a form-fittingconnection between the insert and the surrounding material created bymeans of a cone mold.

This type of manufacturing process thus creates a gearwheel that can beused in particular as an EPAS gearwheel. The gearwheel advantageouslyconsists of a metal hub in the form of a metal insert or insert part,which is surrounded by injection molding with plastic in a moldingprocess (including the gear teeth) to form the gear rim. This eliminatesthe need for expensive and time-consuming finishing of the gear teeth.

Injection molding using a cone mold method creates a high level ofstability on the gear rim because the rim has no seam line and thus nopre-established break-off point.

Thus injection molding is advantageously accomplished using the pieceformed by the cone mold, which is then removed, for example in amachining process. This allows lateral cone mold residue to be removedby machining in a simple work step, in contrast to a multitude ofindividual steps when individual gears are machine-formed in thecircumferential area of the gear rim.

It is also advantageous for the cone gate to be designed with very thickwalls, particularly with a width on the side of the insert of more than60%, particularly from approximately 60% up to 150%, of a maximum wallthickness of the gear rim, in order to be able to fill the gear rimwithout holes or bubbles during injection molding.

Indentations and protrusions in the transitional area between the insertand the gear rim, wherein the gear rim is understood here to refer alsoto a ring-shaped section formed between the actual gears and the insertpart, permit good torsional synchronization and axial force transferbecause of the form-fitting connection with the hub that is createdthereby.

Such a component, including the cone gate, typically also exhibits highaxial stresses. The cone gate pulls the gear rim ring axially in thedirection of the cone gate. As a result, protrusions on the side of thesteel-hub shaped insert opposite to the gate cause the gear rim toexpand and the insert to detach from the gear rim. Thus, it isadvantageous to provide locking mechanisms, particularly in the form ofprotrusions set off-center on the gate side of the externalcircumference of the insert. Preferably only indentations are providedon the side on the insert opposite to the gate, particularly on itsexternal circumference, so that detachments can be avoided to the extentpossible.

The figures below detail an exemplary embodiment. To the extent thatsome of the figures use the same reference numbers, this means that thesame or similarly acting components or functional characteristics areinvolved in each case. The descriptions of other figures can then betaken into consideration. The figures are:

FIG. 1 Components of three manufacturing steps of a gearwheel in aperspective lateral view;

FIG. 2 Components of three manufacturing steps of a gearwheel in alateral sectional view;

FIG. 3 Various perspective views and sectional views of an exemplarygearwheel according to FIG. 1 and FIG. 2;

FIG. 4 A perspective view of an insert according to an embodiment withoff-center protrusions on the external circumference;

FIG. 5 A lateral view and a sectional enlargement of the insertaccording to FIG. 4 in order to visualize such protrusions on thecircumferential area;

FIG. 6 An enlarged sectional view through the insert according to FIG. 4and FIG. 5,

FIG. 7 An enlarged partial sectional representation as a stress diagramthrough a transitional area between an insert with a depression and agear rim injection molded onto it by means of a cone mold;

FIG. 8 An enlarged sectional representation of a stress diagram in theevent of an off-center protrusion on the external circumference of theinsert;

FIG. 9 A perspective lateral view and a sectional view of an insert withprotrusions on the center of the external circumference according to thestate of the art;

FIG. 10 Another sectional representation and a sectional enlargement inthe section of the embodiment according to FIG. 9 with a gear riminjection molded in the circumferential direction according to the stateof the art; and

FIG. 11 A stress diagram of an enlarged section of one such embodimentwith an protrusion on the center of the external circumference of theinsert according to the state of the art.

As shown by FIG. 1, an exemplary gearwheel 1 is manufactured in severalmanufacturing steps. In an initial manufacturing step, an insert 3 thatforms the actual hub is created. The insert 3 is preferably a metal(particularly steel) insert that is used to form, for example, a steelhub that is itself of the prior art.

In a subsequent manufacturing step, this insert 3 is surrounded by meansof injection molding to form a gear rim 2, with a cone mold processbeing used for the injection molding. Accordingly, the center drawingshows the insert completely surrounded by the injection molded plasticin the line of sight. This insert already exhibits a gear structure 21in the circumferential direction. A gear structure 21 of this type canbe cast at the same time. In the cone mold injection process, the insert3 is surrounded by injection molding from the axial direction of an axisX that preferably forms the rotational axis of the gearwheel. The viewshows the side from which the insert 3 is surrounded by injectionmolding, with the center drawing showing an intermediate manufacturingstep in which a cone gate 46 is still in place on the side of theinsert. A sprue 45 is visible in the center or axially.

The cone gate 46 covers a side wall of the insert 3 extending in theaxial direction with a thickness that permits bubble- free injectionmolding even of areas that are remote from the gate. The thickness ofthe casting material in the axial direction lateral to the insert 3 ispreferably more than 60%, particularly approximately 60% up to 150% ofthe maximum thickness of the axial extension of the gear rim 2 that isto be formed. However, depending in particular upon the fluid behaviorof the material used during the cone molding process, a smaller orgreater thickness may be required to achieve an optimal result.

A cone mold also particularly permits the formation of a groove 23 orsimilarly indented structures in an axially extending side wall of thegear rim 2 formed by the injection molding.

The cone gate 46 below is then separated from the side of the insert 3,for example using a machining process. This ultimately produces thegearwheel shown on the right side of FIG. 1, which shows the centralinsert 3 with a central shaft seat, wherein the insert 3 has alreadybeen surrounded by injection molding in the circumferential direction tocreate the finished gear rim 2, including a gear structure 2 and, ifnecessary, groove-shaped structures such as the groove 23.

In a comparison of the left and right drawings in FIG. 1, it is clearthat such a method specifically also permits the manufacture ofgearwheels 1 with a large insert 3. A large insert 3 refers to an insert3 whose external diameter d3 is greater than half of the externaldiameter d1 of the finished gearwheel 1. FIG. 2 shows in a sectionalview three manufacturing steps (arranged vertically) for manufacturingsuch a gearwheel 1. The top drawing shows the insert 3 on a supportingsurface 50 or the lower part of a tool 5. An insert is placed into theshaft seat 31 of the insert 3, which prevents injection molding materialfrom flowing in during the injection molding process. The upper part ofa tool 51 is placed on the supporting surface 50 at a distance above andin the circumferential direction outside of the insert 3. This thereforecreates an injection molding area 54 in which a cone gate 46 is formedduring the injection molding of the gear rim 2 as well as in the axialdirection lateral to a side wall 30 of the insert 3. The upper part oftool 51 has an injection channel 52 for injecting the injection moldingmaterial, wherein the injection channel 52 preferably runs in the centerand in an axial direction to the axis X of the gearwheel to be formed.The upper part of tool 51 and the supporting surface 50 preferablyalready have tool structures 55 that are shaped appropriately so as toform, for example, the groove 23 and a gear structure.

As shown by the center drawing in FIG. 2, therefore, an element that isinjection molded with such a configuration consists of the insert 3,which is surrounded by injection molding by the gear rim 2 in thecircumferential direction and by the cone gate 46 in the axial andlateral direction. The insert 3 has gear tooth structures on its surfacein a circumferential direction that serve to form a form-fittingconnection between the external circumference of the insert 3 and theinternal circumference of the gear rim 2. Shown as geared structures aretwo different types of indentations 32 and 33, which are formed radiallyon the external circumferential wall of the insert 3, and which arefilled with the injection molding material during the injection moldingprocess. An initial indentation 32 is formed, for example, as a grooverunning in a circumferential or rotational direction in order to form astabilizing or retaining gear tooth structure in the axial direction ofthe axis X. This initial indentation 32 is formed off-center on theexternal circumference of the insert 3 and faces the gate sideoff-center on the external circumference of the insert 3.

A large number of secondary indentations 33 are formed parallel to theaxis X and are preferably globoid in shape. After the injection moldingmaterial flows in and hardens, this large number of secondaryindentations 33 causes a form-fitting connection to the gear rim 2,which permits a good transfer of force in the rotational direction.

After separating along a separation line C, for example by means of amachining process, the finished gearwheel 1, which is shown in FIG. 2below, is created.

FIG. 3 shows other views and details of the gearwheel 1 shown in FIG. 2.

FIGS. 4 through 6 show an embodiment that has been modified compared tothe previous figures. Instead of an initial indentation in the form of acircumferential groove (32 in FIG. 1), off-center protrusions 34 areformed on the circumferential surfaces of the insert 3* that areexternal in a radial direction, which after molding engage with thehardened material of the gear rim. At the same time, these protrusions34 are arranged off-center towards a gate side for the gear rim in orderto prevent the molding material from detaching when it hardens duringthe setting and cooling process. Here such protrusions 34 can run in acircumferential direction as shown so as to create a form-fittingconnection between the insert 3* and the gear rim in the axial directionof the axis X.

The insert 3* preferably again has axis-parallel indentations 33* in acircumferential section adjacent to the protrusion 34 that are oblong,for example, and not globoid-shaped in the shown embodiment. At the sametime, the radii of these indentations 33* are selected so that theinjection molding material can flow into them during the injectionmolding process and detachment during the subsequent hardening processcan be prevented completely, or at least to the greatest extentpossible.

FIG. 7 shows a stress diagram for a circumferential section of thetransitional area between the insert 3 and the gear rim 2 afterinjection molding using the cone mold and before a lateral cone gate isremoved. Particularly compared with FIG. 11, which shows an embodimentbased upon the state of the art, FIG. 7 clearly shows an advantageousstress pattern that prevents the hardened plastic material of the gearrim from detaching in a section far from the injection point. Thehardening causes the material, preferably plastic, to pull back only inthe direction of the gate or the gate side, as is clearly shown,although this does not result in any detachment, or any significantdetachment, from the circumferential areas of the insert 3.

FIG. 8 shows a stress diagram of a corresponding section of thetransitional area between the insert 3* and the gear rim 2 of theembodiment according to FIGS. 4 through 6. It is also clear thatshrinking occurs during the hardening or cooling of the plastic mass ofthe gear rim 2, which leads to a displacement on the end facing awayfrom the gate. Nevertheless, fissures do not form, or form only to aharmless extent, when the plastic that constitutes the molding materialdetaches from the adjacent surfaces of the insert 3*.

1. Gearwheel (1) with a gear rim (2) and an insert (3), whose externaldiameter (d3) is greater than half of the external diameter (d1) of thegearwheel (1), characterized in that the insert (3) is surrounded byinjection molding by means of a cone mold.
 2. Gearwheel according toclaim 1, in which the gear rim (2) is formed by means of the cone moldas a component surrounding the insert.
 3. Gearwheel according to claim1, in which the gear rim (2), including an external gear structure (21),is formed by means of a cone mold.
 4. Gearwheel according to claim 3, inwhich the insert (3) has indentations (32, 33) facing outward toward thegear rim (2), into which the material of the gear rim (2) penetrates. 5.Gearwheel according to claim 4, in which at least one part of theindentations is formed as globoid-shaped indentations (33).
 6. Gearwheelaccording to claim 1, in which the insert (3) has off-center protrusions(34) facing outward toward the gear rim (2) which penetrate into thematerial of the gear rim (2).
 7. Gearwheel according to claim 6, inwhich at least one part of the off-center protrusions (34) is formedwith an oblong extension transverse to a rotational direction. 8.Gearwheel according to claim 6, in which at least one part of theoff-center protrusions (34) is arrayed in a displaced configurationoff-center to the gate side of the cone mold.
 9. Gearwheel according toclaim 1, in which the insert (3) is made of metal and the external part(3) is made of plastic.
 10. Gearwheel according to claim 1, in which theinsert (3) is surrounded by injection molding with a metallic material.11. Method for manufacturing a gearwheel (1) with a gear rim (2) andwith an insert (3) whose external diameter (d3) is greater than half ofthe external diameter (d1) of the gearwheel (1), characterized in thatthe insert (3) is surrounded by injection molding by means of a conemold.
 12. Method according to claim 11, in which the gear rim (2) isformed by means of the cone mold as the component surrounding theinsert.
 13. Method according to claim 11, in which a lateral molded partis created by means of a cone mold on an insert wall in a directionaxial to the insert (3).
 14. Method according to claim 11, in which alateral part created in an axial direction by means of the cone mold isdetached.
 15. Method according to claim 11, in which the gear rim (2),including an external gear structure (21), is formed by means of thecone mold.
 16. Method according to claim 11, in which the cone gate isdesigned with thick walls, specifically with a width on the side of theinsert (3) of more than 60%, particularly from approximately 60% up to150% or more, of a maximum wall thickness of the gear rim (2).